avformat/avformat: minor grammar improvement

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avformat.h

@@ -2515,7 +2515,7 @@ void av_url_split(char *proto,         int proto_size,
  * @param ic        the context to analyze
  * @param index     index of the stream to dump information about
  * @param url       the URL to print, such as source or destination file
- * @param is_output Select whether specified context is of input(0) or output(1)
+ * @param is_output Select whether the specified context is an input(0) or output(1)
  */
 void av_dump_format(AVFormatContext *ic,
                     int index,